This is an example of a more general problem with undercharging (or doing spec work) in any creative industry. If you don't charge what you're worth, perhaps because you're trying to be helpful or offering "mate's rates", then you may get one happy client but you'll also get underpaid. Then that client will happily recommend your (excellent value!) services to their friends and colleagues who are also looking for (excellent value!) services and will be happy to similarly underpay you. And so it goes on.
In my experience, there are really only two levels of pricing that work. You can work literally for free, or for some token payment so someone is an official client of your business. This is sometimes helpful very early in a business to start some sort of portfolio and establish a bit of credibility, but otherwise I think it should be reserved for genuine charity work where it's clear that it's a gift and not your normal working practice. Everything else should be charged at decent professional rates immediately, both so you don't undersell yourself and to dissuade potential clients who can't or won't pay decent professional rates and all the other similar potential clients in their network.